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Aug. 18, 1959 R. G. cLAPP COLOR TELEVISION INDEXING SYSTEM 5Sheets-Sheet 1 Original Filed Nov. 3. 1953 Aug. 18, 1959 R. G. CLAPPCOLOR TELEVISION INDEXING SYSTEM 5 Sheets-Sheet 2 Original Filed Nov. 5.1953 M ,www

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United States Patent O COLOR TELEVISION INDEXING SYSTEM Richard G.Clapp, Narberth, Pa., assignor to Philco Corporation, Philadelphia, Ia.,a corporation of Pennsylvania Original No. 2,752,418, dated June 26,1956, Serial No. 390,062, November 3, 1953. Application for reissueMarch 11, 1957, Serial No. 645,388

21 Claims. (Cl. 178-5.4)

Matter enclosed in heavy brackets appears in the original patent butforms no part of this reissue specification; matter printed in italicsindicates the additions made by reissue.

This invention relates to color television systems. More particularlythe invention is described in connection with systems employing cathoderay picture tubes with stripes of different phosphor materials atdifferent elemental screen positions for producing respectively light ofprimary colors red, green and blue in response to impingement of thecathode ray beam.

In systems of this type, having vertically oriented color phosphortriplets, the beam is scanned at a nearly constant speed across thescreen to impinge upon red, green and blue stripes in a periodicsequence. As different colored light emissive phosphor stripes aresuccessively impinged thereby, the beam is modulated in intensity in thesame periodic sequence by corresponding video color signals. Because thecolor of the reproduced light is dependent solely u'pon the position ofthe beam, any instantaneous variation of the scanning speed will resultin a change of beam position causing light production of a colordifferent from that represented by the video signals with which the beamis modulated. Even a small percentage of scanning variation results in alarge amount of color distortion, because of the close spacing of theelemental color stripes which must be so fine that the eye does notresolve the separate colors but -merges the entire display into acontinuous color scene. Even if scanning speed irregularities could beeliminated, the original placement of such fine phosphor stripes wouldhave to be performed with great precision over the entire screen tocause a periodically recurring color pattern matching that of the videocolor signal sequence. Such conditions are so stringent however that itis in some cases convenient to provide color indexing circuits in thesetelevision systems to correct any color distortion caused when the beamis not modulated by video signals of the same color being reproduced.These systems either may select the proper video color signal inresponse to the reproduced color on the screen or may deflect the beamto a color stripe corresponding to the video signal currently modulatingthe beam.

The position of the beam has been detected in prior art indexing systemsby means of indexing stripes specially placed with reference to thecolor phosphors to indicate the position of the beam upon the screen.Should the indexing stripes for any reason not be aligned exactly withthe color phosphor stripes, the beam position information would notproperly indicate the color being reproduced at all screen positions.Therefore it is preferable to detect the color of reproduced lightdirectly from the screen by means of a photoelectric cell sensitive tolight of a particular color. However, the light from the screen isintensity modulated with video intelligence so that the photocell outputsignals include the picture luminance variations. To keep the videosignals from interfering with the indexing function, the systemdescribed in the copending application of D. Sunstein, Serial No.185,106, tiled September 15, 1950, was proposed. The periodic recurrencecomponent of the detected light ice of a particular color is compared inphase with a reference oscillator signal. The reference oscillator.signal synchronously drives a switching circuit which, in this system,selects the video color modulation in a predetermined color sequence.Any phase variations, between the reference oscillator and detectedlight signals, indicate departures from the proper color presentation.,The variations of phase therefore are used in a servo correction loopfor automatic phase control of the oscillator. The phase responsiveindexing system operates independent of the amplitude of the lightcaused by the video intensity modulation.

Although this control action is basically desirable, the indexingsignals which the photocell detects are of low amplitude. These requireamplification before use in a broad-band, high-frequency amplifiercircuit. For example, the frequency for the periodic recurrence ofasingle color is ofthe order of seven megacycles per second when producedfrom a color tube having 450 vertically oriented color triplets of red,green and blue stripes horizontally scanned at constantv speed with arepetition rate of .15,750 cycles per second. It is, however, desirableto effect control for all three reproduced colors. With the combinedcorrection signal derived from photo detectors for the three colors, thefrequency becomes 21 megacycles per second. Therefore a five percentvariation in the horizontal scanning speed will change the colorreproduction recurrence frequency component by more than one megacycleper second. Since the variation may be in either sense, the controlcircuits must be responsive to a'band of frequencies two megacycles Wideat the basis frequency of 2l megacycles in the simplest possible systemutilizing only the fundamental frequency components. The Ibandwidth mustbe greater if sideband energy is also required. Wide band amplifierssuitable for such signals have little gain, and are susceptible tonoise.

In addition, it is difficult to design such amplifiers with a constantphase shift for all frequency components contained in the band to whichthe control circuit is responsive. Therefore phase variations in theamplifier circuit may be reproduced by the correction system to causeindexing errors, since the correction system is sensitive to phase. Foroperation without indexing errors Vtherefore, it is desirable to detecterror signals in a form which can be amplified in a low-frequency,narrowpassband color correction circuit. In this manner the desired gainmay be provided in amplifier circuits of the color correction systemwithout introducing indexing errors because of noise of phase distortionpossible in wide-band amplifiers.

It is, accordingly, a general object of the present invention to provideimproved color indexing apparatus.

Another object of the invention is to provide color indexing systemsfree from the effects of Video luminance level contamination.

Further it is an objective of the invention to provide an indexingsystem which requires low level amplification of only relatively slowlyvarying indexing control signals.

A color television. system embodying the invention therefore includes acathode ray color picture tube with a screen comprising parallel red,green and blue color emitting phosphor stripes arranged in recurringcolor triplets. The cathode ray beam produces light of a particularcolor depending upon the characteristic of the phosphor at the positionon the screen to which the beam is directed. To assure that the desiredcolor is always produced from the color video signals, a color servoloop is provided to modulate the beam with video signals of a particularcolor at the same time light of that color is produced by the screen.Therefore the beam,

3 at any position on the screen, is modulated with the video signals ofthe desired color.

In accordance with the present invention, the color servo loop providesa slowly varying direct current indexing control signal at the lowestlevel. This is effected by using photocell light detectors in a phasecomparison circuit to compare the phase of a reference oscillatorwaveform with that of a periodically recurring light signal from thephosphor stripes. The reference oscillator is used also to synchronize acircuit for sequentially switching video signals of different colors tomodulate the cathode ray beam. Thus any phase variations detected by thephotocells indicate a departure of the color of the reproduced lightfrom the color of the video signals with which lthe beam is modulated.Because of continuous correction action of the color servo loop, theinstantaneous variations during operation of the indexing system aresmall and the band-width required for passing the detected variations isnarrow. Amplification of the detected low level phase variationstherefore is alorded in a narrow-band, low-frequency amplifier circuitwith little phase distortion or etectof noise components lying outsidethe passband, and With high gain. The amplified control signals actuatean automatic phase control circuit connected with thereferenceoscillator. Thus the oscillator phase is automatically corrected in adirection causing selection of video modulation corresponding to @thecolor of the light instantaneously produced by the beam.

Other objects and features of advantage of the present invention will befound throughout the following specification and accompanying drawing inwhich:

Fig. l is a combined block and schematic circuit diagram of a colortelevision `system embodying the invention:

Fig. 2 is a waveform chart illustrating control characteristics affordedby the invention:

Fig. 3 is a graph illustrating the elfect of amplitude upon a phasedetector constructed in accordance with the invention;

Fig. 4 is a graph illustrating operation of the invention inautomatically correcting color errors caused by deviations of rthe beamposition due to changes in the beam scanning speed; and- Figs. 5 to 7are combined schematic and block circuit diagrams of further colortelevision systems embodying the invention.

Throughout the drawings like reference characters are used to designatesimilar component parts to facilitate comparison between the differentviews. Those features which are well known in the art, and whose detailsare not a part of the present invention, are shown in block diagram formAmore readily to point out the nature and construction of the presentinvention.

The circuit diagram of Fig. l illustrates a typical color televisionindexing system operable in accordance with the teachings of the presentinvention. Color pictures are displayed bythe directly viewed colorphosphor screen 9 of a single gun cathode ray beam color tube 10.Picture elements of the dilferent primary colors are reproduced indifferent color Zones represented by the vertically oriented blue, red,and green stripes 12, 13 and 14. The stripes are enlarged for claritybut represent a plurality ofkiine, closely-spaced lines which the humaneye 'may merge into a continuous colorpicture. .Three sets of pairedphotocells 17, 1S and 19 are provided respectively for response to lightof each of the primary colors produced by the corresponding phosphorstripes 12, 13 and 14. Different colors are selectively filtered fromthe color picture on the screen for thispurpose by the respective red,blue and green monochromatic lters 21, 22 and 23. The photocells eachhave viewing angles subtending the entire screen to assure control atall beam positions. Individual color stripes are excited by the beam asit is positioned by the deflection yoke 2 4 and 4 scanning circuit 25 toimpinge upon corresponding color zones.

Video signals from a television transmitter are reproduced in the colorreceiver 27. In this embodiment the video signals are separated intothree simultaneous primary color video components at :the receiveroutput leads 30, 31 and 32. The resulting red, green and blue videocolor signals then serve as one set of inputsignals for actuating thecolor sampling circuit 34. Sampling is effected in the well known mannerby driving the control electrodes of three separate color gating tubeswith the corresponding three color video signals and gating each of thecolors in time sequence by a second set of three sequential gating inputsignals respectively supplied to another control electrode of each tube.The gating signal is derived from the reference oscillator 38 with afrequency such that the color tube beam is periodically intensitymodulated at control grid 35 by a particular color video signal at 'thetime the beam is periodically deilected to a stripe of the same color.Although the color phosphors need not be in theform of vertical stripes,the circuits are simplified when the phosphors are laid out in aperiodically recurring pattern of stripes such that the referenceoscillator 38 serves to periodically gate the video color signals in asequence corresponding to the recurring color pattern of the phosphorsimpinged upon by the bearn as it is scanned in a geometric patternacross the screen 9 at a substantially constant speed.

Since the colors are laid out in repetitive sequences of equal colorareas on the illustrative vertically ruled line color tube screen 9, theoutput signals of the refer nce oscillator 38 may be convenientlydelayed at the three gating leads 39, 40 and 41 by equal phase angles of120. This is effected by operation of the delay lines 43 and ftd. Thevideo signals applied to the color sampling circuit 34 therefore aresequentially gated to appear in a blue, red, green color sequence atIthe picturetube control grid 3S. With the illustrated verticallyoriented stripes, the reference oscillator will operate at a frequencyof about seven megacycles per second with a duty cycle of 33% to providethe Waveform 48 so that each color is gated on for a complete colorelement period of second during each sampling oscillator cycle. Byvarying the phase of the reference oscillator output signals with theautomatic phase control circuit 42, :the relative timing of the colorvideo signals and the corresponding colored light may be altered.Therefore the oscillator phase is controlled to automatically index thecolor video signals in response to the color of light producedinstantaneously by the beam as it impinges upon the picture tube screen9.

In accordance with the present invention, a pair of photocells isprovided responsive to each color and connected in a phase comparisoncircuit to provide a low level indexing signal containing onlylow-frequency variations from a fixed phase relationship. Each photocellis primed by the reference oscillator driving signal 54 to conduct inresponse to light produced during the period that the beam is expectedto impinge upon a stripe of a color to which the photocell isresponsive. This occurs because the photocells are supplied withoperating potentials developed by transformers l5 to 47 at the referenceoscillator frequency. Thus [two high-frequencj.' signals, comprising thecolored light recurrence frequency and the separate electrical phasecomparison driving signal for the photocells, obtained from thereference oscillator 38, are compared directly by conduction of the twop taocelis in each set in opposite sense during respective hair cy-ciesof the alternating oscillator waveforms 54. A resultant low-frequencysignal, indicating only variations in phase of the two signals isdeveloped at resistor 53 from the combined lcurrent of each of thephotocell sets 17, 18 and 19.

The high Ifrequency signal and noise components are by-passed bycapacitor 55 to provide only direct current indexing signals foramplification in the narrowband, low-frequency amplifier circuit 57,wherein high gain may be afforded without introducing phase distortion.Because of the improved sensitivity, the indexing system eflciency isincreased and the cost of the system is decreased, while at the sametime the performance is improved because of greater signal levels andless phase distortion.

The oscillator 38 provides gating signals for both the color samplingcircuit 34 and the photocells 17 to 19 when the system is properlyindexed such that a xed phase relationship is established betweensignals derived from the color of reproduced light and signals switchingthe color of the video signal modulating the picture tube grid 35. Thephase angle magnitude is determined by the electrical characteristics ofcircuit components such as the photocell driving transformers 45 to 47and associaed delay circuits 50 and 51. As will be explained more fullyhereinafter in connection with Fig. 2A, a fixed phase angle other lthanzero between the two referenced signals is desirable in some cases toprovide advantageous operating conditions.

Each set of paired photocells 17 to 19 is connected in a phasecomparison circuit to reproduce signals which are a function of thevariations from the normally fixed phase relationship establishedbetween the driving signal from reference oscillator 38 and therecurrence frequency signal of light produced by the picture tube'screen 9. These signals represent only the low-frequency variations ofphase due to scanning speed nonlinearities and the like. The resistor 53is by-passed by capacitor 55, and the time constant of the resulting R-Cnetwork is chosen to be as great as possible while still allowing theoscillator to follow the low-frequency color phase variations. Ingeneralftherefore, the control signal will be a varying direct currentwhich has high-frequency noise and signal components filtered out by thecapacitor 55.

The indexing signal at resistor 53, which is the sum of the resultantphase departure signals reproduced by the three photocell sets 17 to 19,is preferably chosen to have a zero mean control potential when thesampling oscillator and reproduced colored light signals are at thecorrect phase difference. Any variations of operating potentials lin theindexing circuits will then have less tendency to change the position atwhich the indexing servo system will be locked in, than if it were at apotential which must be maintained by xed circuit conditions and whichtherefore might vary with line voltage lluctuations or the like. This isaccomplished by choosing the fixed phase difference between the twocomparison signals, when the colors are properly indexed, such lthat thephotocells of a paired set conduct the same average currents in oppositesenses during alternate halves of the driving cycle 54 when theyare'active. Therefore variations of phase in either direction willprovide a sensed electrical variation about a locked in null position.

Brightness variations of the light will serve to vary the photocelloutput potential and thus will change the amplitude of the controlsignal developed in the servo loop, but because of the null position abrightness variation will not affect the zero potential of the lock-inposition. As the light becomes brighter, the amplitude of correctionsignals developed in the servo loop becomes greater to effect a morerapid in'dex correction, which is desirable since color errors are morenoticeable to the eye in a. brighter color picture.

Preferably, in order to assure control during blackened out portions ofthe reproduced picture, the photocell sensitivity should be enough sothat there is some degree of control response in the servo loop to thelight produced by beam impingement upon a color stripe when a minimumresidual black signal not seen by the eye is provided. This will preventthe color phase from departing substantially from the correct value atany time, and will prevent color fringing at the edges of black areaswhich might be caused during the time it would take the system tolock-in. This is particularly desired when a single set of photocells isused for a single color as large areas of the picture might be devoid ofthe color to which the indexing system is responsive.

So long as any residual brightness information is present, the singleset of photocells responsive to the color component present will aiforda correction signal. Thus control is assured so long as the reproducedpicture contains a little of the color to which the photocell set isresponsive. However, three sets of photocells for the threecorresponding colors are preferably used to provide more rapid controlaction afforded by a system responsive to the presentation of eachcolor, rather than to a single color of each color triplet. The maximumsensitivity also is substantially increased by addition of the controlpotentials produced by the `different photocell sets.

For a more detailed description of the control action, consideroperation of the phase comparison circuit of Fig. l along with theillustrative waveforms of Fig. 2. Simplication of the explanation isafforded by idealizing the illustrative waveforms and by consideringoperation of a circuit utilizing :a single paired set of photocells. Thereference oscillator gates the alternate photocells of the pair toconduction with the respective excursions 63 and 64 of a waveform ofunity amplitude, such as the square Wave 62 of Fig. 2A.

Sawtooth wave form 69 of Fig. 2B represents the idealized response ofthe photocell to reproduced light of a particular amplitude whencontinuous operating potential is applied to the photocell. Leading edge60 of this waveform results as the beam starts across a color phosphorstripe,building up to maximum amplitude as the beam fully impinges uponthe stripe. Trailing edge 61 represents phosphorescence occurring with alinearized decay after the beam has left the maximum brightnesscondition.

The driving Waveform 62 of Fig. 2A is used to drive the two photocells66 and 67 of Fig. l into alternate conduction during the period in whichblue video signals are gated at the picture tube grid 35. If no bluelight is produced during the photocell driving period, the photocells 66and 67 will conduct equally and in opposite directions so that theresultant current at resistor 53 vw'll b'e zero. However, during thephotocell drive period, the photocells 66 and 67 in addition will beresponsive to such blue light as may occur. Any diierences in thequantity of blue light emitted during the two periods at which ltheportions 62 and 63 of the oscillator waveform are presented will causephotocells 66 and 67 to produce a resultant current which is not zero.This resultant current flow therefore will change as the phaserelationship between the light reproduction Waveform 69 and the drivingwaveform 62 is altered. Consider rst this action in the presence of atypical blue light signal represented by Ithe thick line waveform 69 ofFig. 2B. It is seen that the areas 72 and 73 of Figs. 2C and 2Drespectively are representative of linear multiplication of the twowaveforms 62 and 69 obtained by conduction of the two photocells 66 and67 during their respective gating periods. The algebraic sum of theseareas averaged over the entire period is representative of the potentialdeveloped by integrating the current owing through resistor 53. For thewaveforms shown in Figs. 2C and 2D, the algebraic sum `results in anegative direct current signal level 75. The direct current signal levelis therefore a function of the phase angle between the periodical-lyrecurring components of the electrical excitation signal 62 and thephotocell response signal 69.

The phase of the blue light response signal is changed with respect tothat of the sampling signal by scanning speed variations. A signal ofdifferent phase is illustrated by the waveform 69" of Fig. 2E. It isseen that a positive resultant directy current signal level 78 of Fig.2F is obtained by integrating the areas of waveforms 72' and 73 of Figs.2F and 2G in the same manner. By choosing the phase of the sawtoothwaveform at some position between the two described phase relationshipsof Figs. 2B and 2E, the resultant integrated signal response of thephotocells will be zero. This phase relationship is indicated by thefurther waveform 69" of Fig. 2H. The integration process illustrated byFig. 2l, where the waveform 72" has an area equal to that of waveform73", results in zero potential. Thus the normal operating point at whichthe servo loop is locked in may be conveniently chosen as zero. Thesense or polarity of the resultant control signal with reference to zeropotential therefore indicates the direction of phase departure of theoutput signal developed in response to the blue light from the nominalphase angle of waveform 69". This `information may be used to correctfor color errors by retarding or speeding up the oscillator phase untilthe null position is reached. Although specific idealized waveformshapes have been used to illustrate the operation, the same generalaction will be maintained with substantially different waveforms.

In like manner, the same control action described for the particular setof photocells 17 will be effected for each of the other colors by thecorresponding sets of photocells 18 and 19, so that the total controlpotential at resistor 53 will be additive result of the three developedsignals.

As indicated by a similar analysis of the thin line waveform 70 of Fig.2B, indicative of the blue light of a reduced brightness level, the onlyeffect of changes in brightness level will be corresponding changesinthe arnplitude of the resultant control signals 75 and 78. Thus, withthe lower amplitude of the reproduced color light wave 70, there will bea corresponding decrease of areas of the waveforms 72 and 73.Accordingly the color indexing signals 75 and 78 will also be reducedinamplitude. 'I'he mean control position, however, will stillbemaintained at zero potential since the areas of waveforms 72" 73 in Fig.21 remain equal. Therefore no change of sense results and the brightnesslevel does not affect the lock-in position of the oscillator.

This action may be illustrated by the usual phase comparator responsecharacteristics of Fig. 3, where a dilference in the amplitude of thebrightness represented by the relative positions of the two curvesresults only in the change of amplitude of the control signal and not ina shift of the cross-over position 81. In operation, any suitable phasecomparison technique with this type of characteristic may similarly beutilized to advantage in the servo loop for controlling the oscillatorphase to correct for phase departures. Although generally the oscillatorcontrol circuit 42 is indicated as a phase control circuit, it is notedthat either the frequency or the phase of the sampling oscillator 38 maybe controlled to cor rect for differences of phase between the twosignals derived respectively from the sampling oscillator waveform,serving to gate the video color signals in sampling circuit 34, and thecorresponding signals derived from the light reproduced by the phosphorstripes on screen 9.

In general, the band over which the control is effective is suflicientto correct for variations, shown in Fig. 4, from the optimum deflection'linearity of dotted waveform 83 as illustrated by the typicaldellection sawtooth waveform 84. It may be assumed that deflectionlinearity will not vary at a rate faster than substantially ten timesthe basic deflection frequency. Thus a servo loop which followsvariations up to about 150 kilocycles per second will be sufficient toprovide continuous oscillator correction control under any'reasonablyexpected 'condi-1i `tions due to changes in horizontal deflectioncircuits operating at 15,750 cycles per second. Even though the range ofvariation inthe recurrence rate of vertical color stripes may be of theorder of vtwo megacycles, as hereinbefore explained, control' throughoutthe 15() kilocycle range is 'sufcient to keep the video color samplingcircuit in step with the light produced by the color stripes. Thereforethe necessary bandwidth is considerably reduced by the presentinvention.

Certain variations of the illustrative embodiments may be used withoutdeparting from the principles of the invention. For example, theoscillator may be connected to excite the photocells continuously in adifferent manner, as hereinafter discussed. kAlso other phasecornparator circuits may be utilized, where desired. In any case,however, the photo responsive elements are connected as the activeelements of the phase comparator circuit.

A vertically ruled line screen system having a seven megacyclerecurrence frequency for each color is shown schematically in Fig. 5.Thus the plurality of red, blue and green ruled line color triplets,represented schematically by the heavy labeled lines on the screen 9',may be referenced by the oscillator 38 having a frequency of sevenmegacycles. A single bilateral photo responsive phase'sensitive device19 is excited into periodic conduction by oscillator 38y when greenlight is reproduced on screen 9. The photocell excitation waveform fromoscillator 38 is obtained from lead 86 of delay line 41 in the samephase that the green video signals are sequentially presented to thepicture tube grid 35. It is preferable, if the extra circuit cost iswarranted, to provide separate phase detectors for each of the colors.These may be driven in the proper phase from the respective ends of thedelay line 40 designated at leads 87`and 88. It is not necessary, in allcases, to provide duplicate delay lines for the photocell excitingsignals and the color sampling signals, as'shown in Fig. l. Thus, thetwo waves are referenced in the same phase, as shown in Fig. 5, andsignals from the single delay channel are made both to excite the photodevice 19 and to actuate the color sampling circuit.

Simultaneous color video signals are applied at terminals 30 to 32 toexcite one control grid of each of the three sampling tubes 93, 94 and95. The combination of this video signal with the phased oscillatorsignals from delay lines 40 and 41 at another control grid of thesesampling tubes provides a color video signal at the anode forsequentially actuating the color tube 10 with video signals of differentcolors. The anodes of each sampling tube are coupled to a single loadresistor 97 to present the phased color video signals for coupling byamplifier tube 98 tothe single control grid 35 of the color tube.Separatedpicture luminance and color Video signals may be used in thissystem. Thus the color tube cathode is modulated with the videobrightness signals of circuit 100.

A further embodiment of a color television indexing system isillustrated in the simplilied block circuit diagram of Fig. 6. Thereference oscillator 38 operates in the manner hereinbefore described toprovide a signal for driving two photocells 66 and 67 into conductiononly on the positive signal excursions represented above the photocelllconduction level lines shown in the waveforms 102 and 103. By means oftransformer 105, the oscillator signal phase is split so that thephotocells 66 and 67', connected in the same sense, may be excited onopposite halves of the oscillator cycle. Similar electrodes of thephotocells are thereby connected to opposite ends of the center-tappedsecondary winding 106. Two signal developing resistors 53', andaccompanying by-pass capacitors y55', are connected in series betweenthe grid and cathode of amplifier tube 57 to alford the desired indexingcontrol signals'.

In'thefurth`er' embodiment of Fig; 7, colortube' 10 and referencesignal.

. 9 oscillator servo loop 104 is shown. Ihe remaining portions of thissystem are described in connection with a television signal of the type'shown by the accompanying graph 107. The video signal luminancecomponent is separated 'from the phase-modulated color subcarriercomponent by means of low pass filter 109, and is applied to the colortube 10 by. wayof lead 110 and the video amplifier 111. The color signalis isolated by high pass filter 108 and is Vintroduced to the videoamplifier circuit 111 by way of mixer circuit 116 which converts the3.58 megacycle subcarrier to a seven megacycle subcarriercorrespondingto the color recurrence the vertically-lined color tube 10.

Since the color information must be` synchronized with the transmitter,a rst mixer 113 is provided to reference the color signals with theincoming 3.58 megacycle color subcarrier reference signal at lead 114.This reference signal is derived from a burstsignal incorporated withthe horizontal synchronizing pulses in transmitted signals of the typedescribed. Mixing action of the 3.58 megacycle color reference signaland the4 color indexing signal from oscillator 38 in circuit 113 resultsin a 10.58 megacycle wave. This wave is therefore synchronized with boththe indexing reference oscillator 38 and the transmitter color Thesecond mixer circuit 116 heterodynes the 10.58 megacycle wave with theincoming 3.58 megacycle phase modulated color subcarrier information`from the high pass filter 108. As a result, the desired seven megacyclesignal is produced to excite each color phosphor with a correspondingcolorvideo signal as the beam is deflected thereto by the synchronizeddeection frequency Iof circuit 25. Color sampling is eiectivelyaccomplished at l the tube screen by means of the stripes of diler'entcolors, whichA emit light in time sequence as the beam is scanned atconstant speed across the stripe. In thissystem also', the indexingreference oscillator 38 controls the color indexing action by comparingthe phase of the seven megacycle photocell driving signal with that ofthe reproduced light signal to index the color signals `in the samemanner as in the other embodiments.

It is clear, `from the various embodiments of the invention disclosedand their modes of operation, that an indexing system is provided whichmay be practically used to effect color indexing with a -greatvariety ofcolor tubes and systems. By means of the invention, the color indexingsignals may be produced at low-frequency because of phase comparisonat'the photocell light detector level. This results in a practical,workable system whichv is not responsive tothe amplitude of the videobrightness components and is, therefore, substantially free of videocontamination due to this cause. Those features which are` believedindicative of the nature and scope of the invention are defined withparticularity in the following claims.

I claim:

l. In a color television indexing system including a cathode raytelevision tube having an array of targets individually designated forreproduction of different colors and arranged for excitation by thecathode ray beam at different beam positions, and means connected forboth modulating and scanning the beam periodically across the targets insynchronism with a source of color video signals; the improvementcomprising, color sampling means including an oscillator connected forexciting the beam periodically with varying video signals representingparticular colors as the beam is directed to target positionsdesignating corresponding colors, bilaterallyconductive color-responsivephotocell means responsive to the light produced by impingement of thebeam on designated targets, electrical driving means for synchronouslyexciting said photocell means in a fixed phase relationship with respectto said oscillator at the sampling frequency, and means connected forcontrolling the oscillator phase with resultant signals derived from thephotocell means in response to changes in phase of reproduced colorswith respect to the electrical excitation from said oscillator, thecontrol of the oscillator being in such sense to direct the samplingmeans to excite the beam with only those video signals rprese/nting'thecolor corresponding to that of the target to which the beam is directed.

2. In aV color indexed television system including a beam color tube,means for defiecting the beam of the tube to different colorreproduction zones, and means connected for modulating the beam withcolor video signals including a video colorsampling circuit and anoscillator circuit for driving said color sampling circuit; theimprovement comprising, dual-element photo responsive means forindividually detecting reproduced signals in at least one of said colorzones, means connected for differentially energizing said photoresponsive means in synchronism with the color sampling circuit toproduce a resultant detected signal indicating variations of indexing ofthe beam with different color zones with respect to the videov samplingperiods for the corresponding colors, and oscillator control meansresponsive to said resultant detected signal to control the oscillatorphase and maintain the sampled video color signals in properly indexedrelationship with corresponding color zones.

3. An indexed color television system comprising in combination, a colortube having a beam adapted for deliection to a plurality of differentcolor zones, sampling means connected for modulating the beamsequentially with separate video color signals synchronously with itsdeflection to corresponding ones of said color zones, a referenceoscillator connected to the sampling means to establish the frequency ofsampling the modulation of said beam with the different color videosignals, dualelement detector means connected for excitation by light ofa particular color reproduced by said tube, means for electricallyexciting said detector means differentially with a signal from saidreference oscillator, and phase control means actuatedV byresultantsignals produced by said detector means from interaction of thelight and electrical signal components connected for automaticallycontrolling the oscillator phase in such sense that color registrationerrors are minimized.

4. A color indexing system comprising in combination, dualelementdetector means responsive differentially to light of one color only inpictures reproduced from electrical colorv signals, sampling meansconnected for sequentially reproducing picture light components ofdifferent colors, oscillator means connected for synchronously drivingboth said detector and sampling means in a fixed phase relationship, andoscillator phasing control means connected for indexing the reproducedlight color with the corresponding electrical color signals in responseto resultant signals produced in said detector means from the reproducedlight and the .oscillator drive signal.

5. A color indexing system for a television receiver, comprising, meansincluding an oscillator connected for detecting separate color videosignals from a combined signal havingra phase modulated colorsubcarrier, means for reproducing light of corresponding colors from thedetected color video signals, dual-element photo detection meanselectrically conditioned for differential actuation by reproduced lightof one color only, a phase comparison circuit including the photodetection means for developing output signalswhich are a function ofphase variations between the detected light and the oscillator signals,

and means connected for correcting color errors in refor bothcontrolling the color video signal sequence and electrically primingsaid photocells in dilerential sense for alternate conduction andnon-conduction in response to reproduced light to .produce in saiddetecting means resultant signals, and means responsive to the resultantsignals connected for controlling the oscillator phase to therebyautomatically correct for color errors.

7. A system as defined in claim 6 wherein a iixed phase relationship isestablished between the color video sequence signals and the priming`signals during proper coloi reproduction at such an angle that a zerodirect current error detection signal results when the phaserelationship is held at that angle.

8. A system as defined in claim 6 wherein the means for detecting lightcomprises a pair of asymmetrically conducting photo responsive cellsconnected as active elements in a phase comparison circuit )forproviding said resultant signals.

9. A system as defined in claim 6 wherein the means for detecting lightis connected to provide resultant signals which are substantially theproduct of the waveforms of the reproduced light and the electricalpriming signals applied thereto.

10. An indexing system for a color television receiver comprising a pairof photocells, a picture reproducing device having a striped colorscreen and a scanning beam tor sequentially exciting said stripes toemit light of different colors, means for causing said photocell to beresponsive to light from stripes of one preselected color only, meansfor electrically priming the photocells alternately `for conduction intime sequence when the scanning beam is conditioned to excite stripes ofsaid preselected color, and mean-s directed by photocell output signalsand operative on said electrical priming means to correct colorreproduction errors.

11. A color television system comprising in combination, a source ofcomposite electrical signals for reproducing color pictures, a cathoderay picture tube for producing light of diierent colors at differentcathode ray beam positions, means to deflect the beam to 'beam positionsfor ldifferent colors in synchronism with said signals, mea-ns forintensity modulating the beam with at least a portion of said signals, apair of photoelectric cells, means for differentially applyingenergizing potentials to said photoelectric cells at a recurrence raterelated to that of predetermined components of said signals, opticallilter means selectively admitting to both said photocells light of onepreselected color only developed by sai-d tube in response to saidpredetermined signal components, means developing a control signal fromsai-d photoelectric cells, and means responsive to said control signallfor controlling said recurrence rate.

12. A system as defined in claim 11 wherein said composite electricalsignals comprise 4separate sets of video signals representingintelligence of different colors.

13. A system as defined in claim 12 wherein said predeterminedcomponents comprise said separate sets of video signals, and means areprovided [for sequentially modulating said beam with said predeterminedcomponents at a rate synchronized with said recurrence rate.

14. In a color television receiver, a cathode-ray tube adapted toreconstitute televised images in color, said tube having a screencomprised of a multiplicity of sets of parallel phosphor stripes ofdierent color characteristics, each set having a predetermined colorsequence, means tor scanning the cathode-ray beam of lsaid tubetransversely of said stripes, color sampling means including a referenceoscillator connected to excite said beam sequentially with video signalsrepresenting different predetermined colors as said Ibeam is scannedtransversely of said phosphor stripes, a pair of photoelectric elementsresponsive to the light of one only of said colors, means energizingsaid photoelectric elements from said reference oscillator in alternatephase relation, a load circuit common to said photoelectric elementsttor devel- Cil oping from the combined output of said pair of elementsan error signal indicative of the relative time deviation between theimpingement of said beam on a phosphor stripe of said color and theexcitation ofsaid 4beam by a video signal representative of said color,and means responsive to said error signal for adjusting the phase ofsaid oscillator in a sense to reduce said deviation.

15. The combination claimed in claim 14, wherein tilter means areprovided for substantially eliminating, from sair error signal,frequency components. of the order of the frequency of said referenceoscillator.

`16. The combination claimed in claim 14, wherein lter means areprovided for limiting the bandwidth of said error signal to a bandextending from zero cycles. to a frequency of the order of ten times thefrequency of said cathode-ray-beam scanning means.

-17. In a color television receiver, a cathode-ray tube adapted toreconstitute televised images` in color, said tube having a screen`comprised of a multiplicity of sets of parallel phosphor stripes ofdifferent color characteristics, each set having a predetermined colorsequence, means for scanning the cathode-ray beam of said tubetransversely of said stripes, color sampling means including a referenceoscillator connected to excite said beam sequentially with Vvi-deosignals representing different Vpredetermined colors as the beam isdirected to phosphor stripes of corresponding colors, a pair ofphotoelectric elements responsive to the llight of one only of saidcolors, a load circuit, means energizing said photoelectric elementsfrom said reference oscillator in alternate phase relation, meansconnecting said photoelectric elements and said load circuit as abalanced photoelectric phase discriminator for developing across saidload circuit an error signal indicative of the relative time deviationbetween the impingement of said beam on a phosphor stripe of said colorand the excitation of said beam by a video signal representative of saidcolor, and means responsive to said error signal for adjusting the phaseof said oscillator in a sense to reduce said time deviation.

18. In a three-color television receiver, a single-gun cathode-ray tubeadapted to reconstitute television images in Color, said tube having aiiuorescent scree-n comprised of a multiplicity of parallel-linephosphor triplets of three primary colors, each triplet being arrangedin the same fixed color sequence, means scanning the cathode-ray beam ofsaid tube transversely of said lines in synchronism with the horizontalsynchronizing frequency component of a received color signal, colorsampling means including a reference oscillator connected to excite saidbeam sequentially with video signals representing said primary colors4as the lbeam is directed to phosphor lines of corresponding colors, arst pair of photoelectric cells responsive only to light of the first ofsaid primary colors, a second pair of photoelectric cells responsiveonly to light of the second of said primary colors, a third pair ofphotoelectric cells responsive only to light of the third of saidprimary colors, means energizing said pairs of photoelectric cells fromsaid reference oscillator in three-phase relation, the two photoelectriccells constituting each of said pairs being energized in opposite phaserelationv whereby they are rendered alternately effective to detectlight from said screen, a single load circuit common to all six of saidphotoelectric cells for developing -from the combi-ned output of saidcells. an error signal indicative of the relative time deviation betweenthe generation of light of said colors and the operation of said colorsampling means, and means responsive to said error signal for adjustingthe phase of said oscillator in a sense to reduce the magnitude of saiddeviation.

19. In a color television receiver, color image-reproducing meansadapted to produce light of dierent colors in response to color videosignals, dual-element detector means responsive dierentially to light ofone color produced by said image-reproducing means, oscillator meansconnected to drive said detector means, phase control means responsiveto signals from said detector means to control said oscillator means,and means controlled by said oscillator means for correcting color errorin the color image produced by said image-reproducing means.

20. An indexing system for a color television receiver, comprising apair of photocells, a picture reproducing device having a screen withelements emissive of light of dierent colors and a scanning beam forsequentially exciting said elements, means for causing said photocellsto be responsive to light of one color only, means for electricallypriming the photocells alternately for conduction in time sequence whenthe scanning beam is conditioned to excite elements emissive of light ofsaid one color, and means directed by photocell output signals andoperative on said electrical priming means to correct color reproductionerrors.

21. A color television system comprising in combination, a source ofcomposite electrical signals for reproducing color pictures, a cathoderay picture tube for producing light of dierent colors at dierentcathode ray beam positions, means to deect the beam to beam positionsfor different colors in synchronism with said signals,

References Cited in the le of this patent or the original patent UNITEDSTATES PATENTS Dome Apr. 14, 1953 Lesti Oct. 27, 1953

